Mist elimination hood

ABSTRACT

A mist elimination device in the form of a hood for the removal of moisture from a gas stream treated by a WESP includes a series of gutters on the outer surface of the hood to trap liquid droplets and direct the moisture to channels to drain to the lower region of a housing. Gutters are also provided on the interior surface of the hood to catch liquid droplets carried by the gas stream into the interior of the hood.

REFERENCE TO RELATED APPLICATIONS

This application is a US National Phase filing under 35 USC 371 ofInternational Patent Application No. PCT/CA2004/001037 filed Jul. 16,2004 claiming priority under 35 USC 119(e) from U.S. Provisional PatentApplication No. 60/487,929 filed Jul. 18, 2003.

FIELD OF INVENTION

The present invention is concerned with a mist elimination hood for awet electrostatic precipitator system to remove moisture from a gasstream.

BACKGROUND OF THE INVENTION

Wet electrostatic precipitators (WESP) have been used for many years toremove dust, acid mist and other particulates from water-saturated airand other gases by electrostatic means. In a WESP, particulates and/ormist laden water-saturated air flows in a region of the precipitatorbetween discharge and collecting electrodes, where the particulatesand/or mist is electrically charged by corona emitted from the highvoltage discharge electrodes. As the water-saturated gas flows furtherwithin the WESP, the charged particulates matter and/or mist iselectrostatically attracted to grounded collecting plates or electrodeswhere it is collected. The accumulated materials are continuously washedoff by both an irrigating film of water and periodic flushing.

This type of system is used to remove pollutants from the gas streamsexhausting from various industrial sources, such as incinerators, cokeovens, glass furnaces, non-ferrous metallurgical plants, coal-firedgeneration plants, forest product facilities, food drying plants andpetrochemical plants.

The elimination of free moisture (mist) from the gas stream dischargingfrom the WESP is often provided by mesh pads or chevrons located at theoutlet from the WESP. Both have problems associated with them. Mesh padsare best suited for the removal of entrained droplets fromparticulate-free stream, with high removals being achieved at low micronsizes. However, pads often suffer plugging problems where fiber,particulates and/or VOCs (tars and sublimates) are present in the gasstream. Chevrons provide a high efficiency entrainment separation withlimit drop sizes of 15 to 25 microns, depending on gas velocity andblade spacing, but can also suffer from plugging problems. Theseproblems, in turn, lead to regions of excessive velocity causing dropletre-entrainment and carry-over.

SUMMARY OF INVENTION

The present invention provides a novel mist elimination device whichreplaces conventional mesh pads and chevrons.

In the present invention, the gas exiting the WESP passes downwardlyinto an open-topped housing and over an outer hood surface within thehousing and then upwardly to an outlet communicating with an upperregion of the hood. Liquid droplets accumulate and are drained from alower end of the housing. Such hood structures have previously been usedto attempt to remove the liquid droplets but exhibit considerablemoisture carry-over into the exhaust gas stream, especially underflushing conditions.

In accordance with the present invention, the problems of the prior arthood arrangement are decreased and exhaust gas stream can be providedwith minimal or non-detectable carry-over by providing on the outer-hoodsurface a series of gutters which trap water droplets and direct themoisture to the channels to drain into the lower region of the housing.In addition, gutters are also provided on the interior surface of thehood to catch liquid droplets carried by the gas stream into theinterior surface of the hood to drip down into lower region of thehousing, to the moisture outlet from the housing.

The present invention, in addition to overcoming the problems associatedwith prior hood structures, overcomes the inherent limitations anddrawbacks of mesh pads and chevrons. The mist elimination device of theinvention prevents liquid droplet carryover, both during normaloperation and during flushing of the WESP. No additional mistelimination device is required. The mist elimination device of theinvention requires no maintenance, a considerable advantage over themesh pads and chevrons.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of WESP incorporating a mist eliminatoraccording to one embodiment of the invention;

FIG. 2 is a close-up view of the mist eliminator of FIG. 1;

FIG. 3 is an elevational view of the hood structure;

FIG. 4 is a perspective view from below of the mist eliminator of FIG.1;

FIG. 5 is a detail view of a ring channel;

FIG. 6 is a perspective view of a WESP incorporating a mist eliminatoraccording to another embodiment of the invention;

FIG. 7 is an elevational view of the mist eliminator hood of FIG. 6; and

FIG. 8 is a view from above of the mist eliminator hood of FIG. 6.

DESCRIPTION OF PREFERRED EMBODIMENT

In the drawings, preferred embodiments of the invention are illustratedby way of Example. It is to be expressly understood that the descriptionand drawings are only for the purpose of illustration and as an aid tounderstanding, and are not intended to be a definition of the limits ofthe invention.

Referring first to FIGS. 1 to 5 of the drawings, FIG. 1 shows a WESPinstallation 10 modified to include a mist eliminator 12 constructedaccording to one embodiment of the invention. The WESP installation isof conventional construction comprising vertically arranged comprisingdischarge electrodes and collecting electrodes. Any desired arrangementof such elements may be employed, including square tube, round tube,hexagonal tube or plate. A moisture-laden gas stream to be treated isfed through an inlet header 14 to the upper inlet to the WESP downthrough the tubes containing the electrodes to the lower outlet 16.

Connected to the lower outlet 16 is the mist eliminator device 12provided in accordance with one embodiment of the present invention. Themist eliminator device 12 includes a chamber 18 having a sloped bottomwall 20 to a moisture outlet 22. Inside the chamber 18 is a hood 24comprising upwardly sloping walls 26.

The mist eliminating device 12 accelerates the gas flow entering thedevice, causing droplets of free moisture to accelerate downward andthen break free of the gas stream as the gas stream turns approximately180 degrees into the interior of the hood 24 and from there to the gasoutlet 28 from the chamber.

A problem of moisture moving down the outer wall 26 of the hood 24 anddripping off the edge of the hood 24 to be swept into the exiting gasstream, is eliminated by providing a series of drip rings or guides orgutters 30 on the external walls 26 of the hood 24. The gutters 30 aredownwardly sloping from an apex to guide the moisture on the hood to thesides of the walls and then to drains 32 from which the accumulatedmoisture drops into the lower portion of the chamber 18 to the moistureoutlet 22.

To capture any moisture which may be swept into the gas stream, furthergutters 34 are provided on the interior wall of the hood 24. In order tocapture any moisture which may remain or accumulate on the inner wallsof the gas outlet 28 and which is swept along by the gas stream, a ringchannel 36 may be provided on the inner wall of the outlet 28 with anycollected moisture draining through drain 38 to the moisture outlet.

The gutters 30 may also provide structural support to the hood 28.

Turning now to FIGS. 6 to 8, there is shown thereon an embodiment of theinvention in which the WESP is of circular cross-section. The samereference numerals are utilized to identify the equivalent parts.

EXAMPLES Example 1

This Example illustrates the results obtained using a laboratory scaleWESP structure.

A ⅛ scale model of a commercial rectangular cross-section WESP structurewas set up as illustrated in FIGS. 1 to 5. The model extended from theWESP inlet, through the collection tubes and into the outlet hood andduct. The humidification spray nozzle was simulated using a singlemulti-orifice air atomized spray nozzle operated at 50 to 60 psi toensure good atomization and fine droplet size.

In the absence of the gutters provided in the hood, there was strongvisual evidence of a significant amount of water being carried into theoutlet duct, particularly during a tube wash operation. However, withthe addition of the water collection gutters to the outside of the hood,the amount of liquid observed to be carried over was significantlyreduced.

The liquid carryover was determined by droplet counters and the averagecarryover was 0.000764 US gpm/ft². With the wash spray on, the carryoverwas increased to only 0.00542 US gpm/ft².

After several minutes of operation, liquid began to accumulate on theduct walls and ran along the surface, not being measured by the dropletcounters in the outlet duct A ring channel was added to the interior ofthe hood to capture the accumulated liquid from the duct walls.

Example 2

This Example illustrates the results obtained using a further laboratoryscale WESP structure.

A ⅙ scale model of a circular cross-section WESP structure was set up asillustrated in FIGS. 6 to 8, with the gutters in place. The modelextended from the horizontal inlet duct through the WESP vessel to theoutlet duct. The humidification spray nozzle was a singlemultiple-orifice air-atomized spray nozzle operated at 50 psig to ensuregood atomization and fine droplet size with a mean of 27 microns.

The WESP flush sprays were simulated by a 8 Bete WL-1/4 60 degreehydraulic nozzles mounted on a ring header at equal spaces. During washsimulation, the header was operated at a flow rate of 6 usgpm atapproximately 10 psig.

The liquid carryover to the outlet duct was determined by dropletcounters. With only the inlet spray operating, the carryover was4.43×10⁻¹⁰ usgpm/ft² with an average droplet size in the outlet duct of3.2 microns. With only the flush sprays operating, the carryover was6.06×10⁻⁸ usgpm/ft² with an average droplet size of 20.6 microns. Withboth sprays operating, the carryover was 9.07×10⁻⁹ usgpm/ft² with anaverage droplet size of 12.1 microns.

There was little airborne water observed entering the outlet ductwork,as seen from these measurements. Any liquid running along the wall ofthe outlet duct, resulting from droplet impingement on the duct wallsand condensation, was captured by an additional gutter or collectionchannel at the outlet.

Example 3

This Example illustrates the results obtained using a plant scale WESPstructure.

A rectangular full scale mist eliminator hood structure as illustratedin FIGS. 1 to 5 was installed to replace an existing mesh padarrangement which was causing operational problems due to plugging andfree moisture carryover from the WESP.

The installation reduced moisture carryover to values below detection asmeasured using EPA Method 5 and has been operating for more than eightmonths without maintenance or interfering with production.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a novelmist elimination hood for a WESP system that removes specific amount ofmoisture from the gas stream while avoiding the problems of plugging andmaintenance associated with the most commonly-employed mist eliminationsystems. Modifications are possible within the scope of the invention.

1. A method for removal of moisture from a gas stream, which comprises:passing said gas stream to a mist eliminator device comprising: achamber having an upper inlet for said gas stream, a lower outlet forcollected liquid and a second outlet for product gas stream, a hoodlocated generally axially in said chamber having walls slopingdownwardly from an upper crown to a lower extremity defining an exteriorand a space beneath the hood, at least one moisture collection channelprovided on the exterior surface of said walls positioned to collectliquid on said walls and guide the collected liquid to at least one flowchannel positioned to direct collected liquid to below the hood, atleast one moisture collection channel provided on an interior surface ofthe walls positioned to collect liquid entrained in the gas stream andpassing from the exterior of the hood into the space beneath the hood,and an outlet duct communicating with the space beneath the hood andjoined to the second outlet for guiding gas entering the space beneaththe hood to the second outlet, passing said gas stream downwardly fromsaid inlet into engagement with the exterior surface of the walls,collecting moisture from the exterior surface of the walls in said atleast one moisture collection channel provided on the exterior surfaceof the hood and guiding said collected liquid to said at least one flowchannel and to below the hood, removing collected liquid guided to belowthe hood from said lower outlet, passing said gas stream into the spacebelow the hood and collecting extrained moisture from said gas stream bysaid at least one moisture collection channel provided on the interiorsurface of the hood, and passing said gas stream to said second outletthrough said outlet duct as said product gas stream.
 2. The method ofclaim 1, wherein said chamber and said hood have a rectangular or squarecross-sectional shape.
 3. The method of claim 1, wherein said chamberand said hood have a round or oval cross-sectional shape.
 4. The methodof claim 1, further comprising an additional moisture collection channelpositioned on an interior surface of the outlet duct and collectingliquid running along the interior surface of the outlet duct.
 5. Amethod for the treatment of a gas stream containing particulates andsaturated with moisture, comprising: passing said gas stream to a mistelimination precipitator assembly comprising: an upper inlet and a loweroutlet, electrostatic precipitator elements extending from the upperinlet to the lower outlet for removing contaminants from the gas streampassing from the upper inlet to the lower outlet, means for flushingand/or irrigating the electrostatic precipitator elements, and a mistelimination device in unobstructed fluid flow communication with thelower outlet for removing entrained droplets from the gas stream by themethod as claimed in claim 1; and passing said gas stream through theelectrostatic precipitator elements from said inlet to said outlet.